1. Field of the Invention
The present invention relates to apparatus and methods pertaining to ball grid array technology and, more specifically, to apparatus and methods for filling a ball grid array with solder balls.
2. Description of the Related Art
The electronics industry is one of the most dynamic and important industries today. It has literally transformed the world and provides many products that affect our daily lives, for example, telephones, television, personal computers, cellular phones, pagers, video camcorders, audiovisual products, etc. One of the key technologies that helps make these products possible is electronics packaging. This field of technology can be divided into a hierarchy of levels beginning with chip level packages and proceeding to multi-chip packages, printed circuit boards, mother boards, and component cases including boards, power supplies, etc.
A key area of development in the field of electronic packaging is the area of chip level packaging and interconnections. The most common types of chip level interconnections are wire bonding, tape-automated bonding (TAB), and solder bumping. Among these three technologies, solder bumped flip chip provides the highest level of packaging density with the least package space. The solder bumping is created by solder balls which are reflowed onto connection points or pads on the chip and/or the package. The solder balls are arranged in arrays on the chips and the packages. These arrays are known as ball grid arrays ("BGA").
Ball grid array packaging is rapidly emerging as the technology of choice for high input/output (I/O) count integrated circuits (IC's). Ball grid arrays deliver higher density and yields than traditional packages without requiring fine-pitch processing or new assembly equipment. Driven by the increasing I/O as IC's become larger and more complex, the demand for ball grid array packages is expected to grow from fewer than 20 million units in 1995 to more than 2 billion by 1999.
In BGA technology, the solder balls must be placed onto desired locations in an array on the substrate. "Substrate" as the term is used herein refers to the item upon which the solder balls are to be placed and bonded to form the ball grid array. Examples of such substrates include printed circuit boards, flux circuits, and ceramic panels. They may be housed in the form of individual chips or boards, or held in carriers such as boats or trays.
Each substrate typically includes a plurality of solder sites or pads positioned in an array corresponding to the desired pattern of bonding sites for the substrate. The solder pads typically comprise a metal pad, for example, measuring slightly smaller than the solder ball which is to be placed upon the solder pad.
The solder balls typically used in BGA technologies generally comprise eutectic solders such as tin-lead solders, solder coated copper, or high temperature alloys. Examples of tin-lead solder ball compositions are 63% tin--37% lead and 62% tin--36% lead--2% silver, typically for use with plastic ball grid arrays, and 10% tin--90% lead, typically for use with ceramic ball grid arrays, although other compositions are possible. The solder balls used in current applications typically have sizes ranging from as small as 5 mills (thousandths of an inch) to as large as 30 mills. Other sizes and size ranges have been used in the past and quite well may be used in the future. In a given application, a single size of solder ball is commonly used, although this is not necessarily true universally. As used herein, the term "solder ball" is used to refer to the generally spherical unit of bonding material used in the ball grid array to bond and electrically couple the solder pads or bonding sites on the substrate to an object to which the substrate is to be electrically coupled.
It is generally a necessity for high-quality ball grid array technology that the solder balls be accurately placed on the solder pads. In most applications, it is also a requirement that one and only one solder ball be placed on each solder pad. Moreover, this precision placement must be done in a way which preserves the phyiscal and geometric integrity of the solder ball. It is generally unacceptable, for example, for any deformation of the solder balls, slicing of the solder balls, etc. to occur. When a solder ball is cut in two during processing, for example, it is generally necessary to stop the process, inspect the solder ball reservoir for ball fragments, and to remove those ball fragments before processing can continue.
The placement of the individual solder balls onto a substrate in the ball grid array pattern traditionally has been done using a stencil or template. The template includes a number of holes positioned in a pattern corresponding to the solder pads on the substrate. Each hole is slightly large then the diameter of the solder balls, so that one solder ball easily slides or falls into each hole.
The substrate generally has been previously processed so that a flux has been applied to the solder pads. The flux interacts with the solder of the solder balls in known fashion to facilitate bonding, for example, to act as a pad cleaning and wetting agent.
The substrate and/or the template are moved relative to one another so that the substrate contacts the template, and the array of holes in the template are aligned with the solder pads on the substrate. Solder balls then are distributed so that one solder ball is placed into each of the holes. The solder balls thus rest upon and are in contact with the respective solder pads. The template then is removed, and the substrate, including the properly-positioned solder balls, are further processed, such as by placing them into a reflux oven to melt the solder balls and thereby bond the solder balls to the solder pads.
A number of methods and apparatus have been developed for distributing the solder balls into the holes of the template, and otherwise for the handling, control and placement of the solder balls. U.S. Pat. No. 5,499,487, issued to McGill on Mar. 19, 1996 and U.S. Pat. No. 5,551,216, issued to McGill on Sep. 3, 1996, provide examples. In these patents, a device is disclosed which comprises an apparatus for placing solder balls in a ball grid array. The apparatus includes a wheel having an inner and outer face, wherein the wheel is rotatable about a horizontal axis. The apparatus includes means for attaching a ball grid array to the inner face of the wheel, means for attaching a tooling fixture to the outer face of the wheel in position corresponding to that of said ball grid array, means for forming a reservoir of solder balls at the bottom of the wheel, and means for controllably rotating the wheel to move the tooling fixture through the reservoir in a manner to fill recesses in the fixture with solder balls and to remove from the surface of the array any excess solder balls which are not occupying recesses. The inner and outer faces are separated a distance to permit the tooling fixture to engage solder balls in the reservoir while the ball grid array does not engage the solder balls.
The solder balls on the ball grid array often are very closely spaced and the recesses into which they must be located are very small. Because of the high volume requirements and the highly competitive nature of the electronics industry, the placement of the solder balls on the ball grid arrays must be performed in a minimum amount of time, and for as little cost as is feasible. The handling, control and placement of the solder balls often is a critical portion of this operation.
Key factors in designing a solder ball placement apparatus and method include the speed at which the placement can be accomplished, the accuracy and yield of the process, the cost of the equipment to perform the process, the reliability of the equipment, etc. Considering the extremely small size of these solder balls, which can range in diameter down to 5 mils and potentially smaller, the high number of solder balls required for the ball grid arrays to interconnect today's high density chips, and the precision required in the placement of the solder balls, achieving high yields at high throughput rates has been difficult.
One limitation of known ball placement devices and methods has been that they do not always effectively place solder balls in all available template holes. Another limitation has been that such known devices and methods generally have relied on gravitation force to maintain positive control over the solder balls once they are in the template holes. Using such devices and methods, the balls may become dislodged or otherwise be removed from the holes. In some instances, for example, known techniques for cleaning the top surface of the template to remove excess solder balls has resulted in dislodging balls from inside the holes.
Another limitation of some known devices has been a difficulty in removing solder balls from the template holes to place them on the substrate. Solder balls sometimes became lodged in the holes and do not readily come out when desired.